Integration application question?

2013-05-19 3:34 am
Question from NSS Mathematics in Action Module 1 Supplementary Exercises.

The rate of change of the sales(in hundred thousand per year) of mobile phone model F40 can be modelled by f(t)=(10t)/(2t^(2)+1)^(2) after t years.

(a)Find the total sales of mobile phone model F40 in the second year.
(b)If the total sales in the previous year are less than 10 000, the mobile phone will no longer be sold in the market. Determine whether mobile phone model F40 will be sold or not in the fifth year.

Answer from the book: (a): 56 thousand (b): no

Thank you so much!

回答 (2)

2013-05-20 11:51 pm
✔ 最佳答案
Lets represent Sales by S
Then rate of sales change = dS/dt = f(t) = 10t/(2t^2+1)^2 ---- (1)

From this the goal would be to obtain S = S(t)
This can be done using numerical approximation or by finding exact solution using substitution.

First exact solution.
Substitute Z = 2t^2 +1 ..... (2)
which mean dZ = 4t dt .... (3)
From equation (1), (2) and (3) we obtain
dS=(5/2)*(Z^-2)dZ ... .(4)
Direct integraton results in
S = (-5/2Z)+C ... (5) where C is integration constant
From equation (2) and (5) we have
S = (-5/2*(2t^2 +1))+C .... (6)

But at t =0, there are no sales and hence S =0 (initial condition)
Substituting initial condition in (6), we have the equation for sales as

S = 2.5 (1-1/(2t^2 +1)) ..... (7)

Calculating sales at end of every year and hence sales per year, yeilds following table

t (years) Total Sales *1E5 Sales in Year *1E5
1 1.67 1.67
2 2.22 0.56
3 2.37 0.15
4 2.42 0.06
5 2.45 0.03
6 2.47 0.01

This shows that
(a) Around 56000 models are sold in year 2
and
(b) The model will not be sold in year 5 as the sales in year 4 are below 6000 (and hence < 10000)


Second Numerical Approach:
I simply made an excel function for equation (7) using first order backward difference scheme using a step size of 0.001 years. This would also yield the same result as above. See excel VBA function below..

Function TotalSalesForYears(t As Double) As Double
Dim Step, time, Slope, TotalSales As Double
Step = 0.001
time = 0
TotalSales = 0
Do
Slope = 10 * time / ((2 * time * time + 1) ^ 2)
TotalSales = TotalSales + Step * Slope
time = time + Step
Loop While (time <= t)
TotalSalesForYears = TotalSales
End Function
參考: reference: Introductory methods in numerical analysis. s.s.sastry. prentice hall 1998
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